Non-coplanar symmetric (e, 2e) momentum profile measurements for helium: an accurate test of helium wavefunctions

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Abstract
The 1200 eV non-coplanar symmetric (e, 2e) cross sections for helium leading to the n=1, 2 and 3 ion states were measured with a modified coincidence spectrometer using position-sensitive microchannel plate detectors. The ground-state momentum profiles (cross sections) are compared with the momentum distribution given by the Hartree-Fock wavefunction, the accurate correlated wavefunction of Joachain and Vanderpoorten (1970), and the momentum distribution derived from accurate Compton profile measurements by Lee (1977). Good agreement is obtained between theory and the (e, 2e) measurements, but the Compton momentum distribution is seriously in error at low momentum. The small discrepancy at high momenta between the present measurements and the plane-wave impulse approximation is completely explained by allowing for distortion of the continuum electron waves by the helium and ion potentials. The n=2 and 3 cross sections cannot be explained using the helium HF wavefunctions, but are well described by the accurate correlated JV wavefunction. The measured n=2 to n=1 cross section ratio discriminates between several accurate correlated helium ground-state wavefunctions.